抄録

Here, we study the influence of cross-linking density, $X$, on the swelling, thermal and electric field response of trifunctionally cross-linked liquid single crystal elastomers (V3 LSCEs) swollen with low molecular weight liquid crystals, 4-$n$-pentyl-4-cyanobiphenyl (5CB). The cross-linker concentrations are $X_{\text{3V3}}=3.3$ mol %, $X_{\text{5V3}}=5$ mol %, and $X_{\text{7V3}}=7$ mol %. LSCE networks are characterized by frozen-in orientation order, $P$. $X_{\text{c}}$ is a constitutional critical point for LSCE networks. When $X<X_{\text{c}}$ ($P=0$), there are no shape changes and a nematic-isotropic transition takes place at $T_{\text{NI}}$. When $X>X_{\text{c}}$ ($P>0$), supercritical behavior prevails over a range of temperatures, $\Delta T_{\text{s}}$. Taking $\Delta T_{\text{s}}\propto(X-X_{\text{c}})^{1/2}$, we find a critical cross-linking density, $X_{\text{c}}\approx 3$%, for V3 LSCEs. 5CB diffuses ${\perp}\mathbf{n}$ into V3 similar to isotropic gels with typical times, $\tau_{\perp\mathbf{n}}$ in minutes, that decreases with increasing $(X-X_{\text{c}})$. Swelling with reorientation effects is initiated by 5CB propagating into V3 followed by relaxation of the front profile with $\tau_{\bot}^{(2)}$ (twist), and $\tau_{\|}^{(13)}$ (splay-bend) that both increase nearly parabolically with increasing $(X-X_{\text{c}})$. Front speeds are 50% faster in 5V3 than in 7V3 but no fronts were observed in 3V3. Compared to dry V3 LSCE volumes, $V_{\text{d}}$, the swollen volumes, $V_{\text{s}}$, increased as $X\rightarrow X_{\text{c}}$: $V_{\text{s}}/V_{\text{d}}\sim(X-X_{\text{c}})^{-0.27}$ reducing the frozen-in orientational order at $X$ to $\tilde{X}=X(V_{\text{d}}/V_{\text{s}})$. The maximum electromechanical effect found in swollen V3 was $\delta_{\text{MAX}}\sim(\tilde{X}-\tilde{X}_{\text{c}})$ with $\tilde{X}_{\text{c}}\sim 0.25$%. The size of the effect is much smaller than observed in 8A2 which has a larger $P$. For 7V3 the effect is about 4 times smaller and for 5V3, it is an order of magnitude smaller.